This invention relates to echo cancellation and, more particularly, to a subband echo suppressor.
In the cancellation of echoes either in telephone systems, acoustic systems or the like, it is desirable to minimize any residual echoes remaining after adaptive echo cancellation. To this end, echo suppressors are often employed as post-processors to adaptive echo cancelers. Of particular interest are so-called subband echo suppressors. In one such echo suppressor, the bandwidth of the voice channel is divided into a plurality of contiguous subbands by employing a bank of passband filters. The output of each filter is center clipped and, then, filtered to remove clipping distortion. The attenuation in each subband is adjusted to continuously reset the clipping level to just remove the entire echo signal in the subband. That is, the clipping level is such that the echo peaks are directly suppressed to zero. See for example, an article authored by O. M. M. Mitchell and D. A. Berkley entitled xe2x80x9cA Full-Duplex Echo Suppressor Using Center Clipping,xe2x80x9d Bell System Technical Journal, Vol. 50, pages 1619-1630, 1971 and an article authored by M. M. Sonhi and D. A. Berkley entitled xe2x80x9cSilencing Echoes On The Telephone Network,xe2x80x9d Proceedings of the IEEE, Vol. 68, No. 8, pages 948-963, August 1980.
More recently, a subband echo suppressor has been proposed in which a bulk signal gain value for the echo path is generated and used to adjust the attenuation in all the subbands. The threshold of echo suppression engagement is matched to the bulk gain value in the echo path and used in all the subbands, and the suppression attenuation is adjusted uniformly in each subband in response to the bulk gain value whether or not there is any echo component to suppress or there is a larger than average echo component to suppress in any of the subbands.
These and other problems and limitations of prior known echo suppressors are addressed in a subband echo suppressor by individually, controllably adjusting the echo suppressor attenuation level in each of the subbands to gradually attenuate echo peaks having a magnitude lower than a clipping threshold with the magnitude of the attenuation increasing for residual echoes of decreasing magnitude. Additionally, the threshold of suppression engagement is dynamically matched to the signal gain in the echo path for each subband. If the signal gain in a subband is high (strong echo), the suppressor must attenuate residual echo peaks in the subband of higher energy to maintain a constant quality of service.